This invention relates to a spray gun for applying coatings such as paint in which compressed air is used both to atomise the paint to form the spray and to control the shape of the spray pattern emerging from the nozzle of the spray gun.
Spray guns for spraying coatings such as paint, using compressed air to atomise the paint and control the shape of the resulting spray pattern, are well known. Examples of such spray guns are disclosed in, for example, U.S. Pat. Nos. 5,803,367 and 5,209,405.
Spray guns of this type typically have a spray head comprising a nozzle assembly and an air cap. The air cap has a central aperture which surrounds the paint-dispensing tip of the nozzle, such that air passing through the central aperture past the nozzle tip serves to atomise the paint to form a spray. The air cap is also provided with forwardly extending formations known generally as xe2x80x9chornsxe2x80x9d which direct compressed air inwardly towards the plume of atomised paint emerging from the central aperture so as to shape the plume of atomised paint to a desired pattern, such as an elliptical pattern. Usually a pair of horns are provided and these are generally arranged symmetrically with respect to the central aperture. Air used to shape the spray pattern is often referred to as xe2x80x9cfan airxe2x80x9d whereas air used to atomise the paint is often referred to as xe2x80x9catomisation airxe2x80x9d or xe2x80x9catomising airxe2x80x9d.
In order to provide a supply of compressed air to both the central aperture and the horns, the incoming air must be partitioned, and in known types of spray gun, complex baffle arrangements are often employed in order to divide the incoming air stream in a consistent and controlled manner and ensure that the required pressures of atomisation air and fan air are delivered to the central aperture and horns respectively. The complexity of the baffle arrangements, and the number of parts required, can considerably complicate the manufacture of the spray guns.
It is an object of the present invention to provide a simplified means of controlling the partitioning of compressed air between fan air and atomisation air and to simplify the manufacture of the spray guns, for example by reducing the number and complexity of the parts required in the spray head.
The present invention provides an improved spray gun in which a simple annular sealing member is used to partition incoming air between atomisation air and fan air. The interior of the main body of the spray gun is configured to provide a plurality (e.g. two) of air inlet passages leading from the compressed air source (usually via an air control valve) to an air distribution chamber within the spray head, and air pathways are provided within the spray head for directing air onwards to both the horns and the central aperture. The annular sealing member serves to divide the air distribution chamber such that air passing into the chamber to one side of the sealing member is directed to the horns whilst air passing into the chamber to the other side of the sealing member is directed to the central aperture.
Accordingly, in a first aspect, the invention provides a spray gun comprising a main body, a spray head, an inlet for a liquid to be sprayed, and an inlet for a supply of compressed air;
the spray head comprising an air cap, a hollow nozzle member having a nozzle tip for dispensing the liquid, and means for securing the air cap and nozzle member to the main body;
the air cap having a central aperture through which atomising air is directed to atomise liquid dispensed through the nozzle tip to form a spray, and one or more lateral apertures through which fan air is directed for shaping the spray to a desired pattern;
the hollow nozzle member having a radially extending weir which together with a surface of the main body defines an air distribution chamber for receiving and distributing compressed air from the supply inlet;
wherein an annular sealing member having an annular wall is sealingly disposed between the radially extending weir and the said surface of the main body so as to divide the air distribution chamber, whereby air passing into the chamber thorough an opening to one side of the annular wall is directed to the central aperture of the air cap and air passing into the chamber through an opening to the other side of the annular wall is directed to the lateral apertures of the air cap.
The annular wall can be circular or non-circular, for example ovoidal or polygonal (e.g. octagonal) but preferably it is circular. The annular wall is preferably set into a recess in the said surface of the main body. This provides a simple means of locating the sealing member and ensuring that it remains in place without the need to provide additional securing means. The recess is advantageously formed by a simple machining operation into a leading surface of the main body of the spray gun. Alternatively, the recess can be formed in the surface during casting of the gun body.
For ease of manufacture, the main body of the spray gun is formed (e.g. machined) to provide a plurality (e.g. two) of air passages opening into the air distribution chamber wherein the openings are arranged symmetrically with regard to a main axis of the spray gun. In such a case, the annular wall of the sealing member may advantageously be arranged eccentrically with regard to the main axis, such that one of the air passages through the main body opens into the air distribution chamber within a region bounded by the annular wall, whilst the other symmetrically disposed air passage opens into the air distribution chamber outside the region bounded by the annular wall of the sealing member. Air from within the region bounded by the annular wall of the sealing member can then be directed onwards to the central aperture whilst air outside the region bounded by the annular wall of the sealing member can be directed onwards to the horns, or vice versa.
Typically, air from the air distribution chamber is directed to the central aperture via a path extending through the weir, whereas air is directed to the lateral apertures (horns) via a path extending around the edge of the weir. Thus the weir may be provided with one or more holes to allow movement of air between the air distribution chamber and the central aperture of the air cap.
The air cap preferably has a rearwardly extending cylindrical formation which, together with a forward surface of the radially extending weir defines an annular chamber surrounding the hollow nozzle member for receiving atomising gas from the air distribution chamber. The central aperture is typically defined by an opening in a front wall of the annular chamber, and the nozzle tip will usually protrude into the central aperture so as to define an annular flow path around the nozzle tip for the atomising gas. Air passing along the annular flow path past the nozzle tip will bring about atomisation of the paint or other coating liquid as it emerges from the nozzle tip.
The sealing member is typically formed from a plastics material, the plastics material being such that at least the edges of the annular wall are crushable or deformable to allow a seal to be created between the main body of the spray gun and the weir of the nozzle assembly. However, the sealing member should also be sufficiently rigid to be able to retain its shape and not collapse or deform to any extent under the pressure of air entering the air distribution chamber. Thus preferred plastics materials are relatively rigid plastics such as polyacetal or polyamides, or polyolefins such as polyethylene and polypropylene. A currently most preferred plastics material is high density polyethylene.
A major advantage of the sealing member is that not only does it provide a means of partitioning the air distribution chamber and dividing the flow of compressed air between atomisation and fan air, but it also provides a seal between the main body of the spray gun and the nozzle assembly thereby avoiding the requirement for a separate sealing member. The number of parts required in the spray head is thereby reduced and hence manufacture is considerably simplified.
The invention has been defined and described so far with reference to the distribution of air within the spray head. However, for the avoidance of doubt, the term xe2x80x9cairxe2x80x9d as used herein includes not only air as such but also any other gas that might conceivably be used for atomisation and spray pattern control in a spray gun.
The invention will now be illustrated and described in more detail, but not limited in any way, with reference to the particular embodiment shown in the accompanying drawings.